Sawtooth waveform generators



SAWTOOTH WAVEFRM GENERATORS Hugh Lyon Mansford, Uxbridge, England, assignor to Electric & Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Application January 31, 1956, Serial No. 562,557

Claims priority, application Great Britain February 3, 1955 8 Claims. (Cl. 331-145) This invention relates to sawtooth waveform generators.

In Oscilloscopes it may sometimes be desirable for the duration of the return strokes produced by the sweep circuit to be variable independently of the duration of the forward strokes, so that a variable delay can be introduced prior to the visible trace produced by the forward strokes, or alternatively so that two vertically spaced traces can be made visible, one corresponding to the forward strokes and the other corresponding to the return strokes. The latter facility may be required for displaying a repetitive signal on two different time scales.

One object of the present invention is to produce an improved sawtooth Waveform generator which, though of relatively simple construction, enables the duration of the forward and return strokes to be controlled substantially independently of each other.

According to the present invention there is provided a sawtooth waveform generator comprising an amplifier, a capacitor, and a switch having one condition in which said capacitor is operative to provide negative feedback from the output electrode to the input electrode of said amplifier to cause the generation of a forward stroke of a sawtooth waveform by integration in said capacitor, and having another condition in which said capacitor is inoperative to provide negative feedback and said amplier is operative to discharge said capacitor to produce a return stroke of said waveform.

It will be appreciated that by varying, on the one hand, circuit parameters which control only the functioning of the amplifier during integration, that is when acting as a Miller integrator, and by varying on the other hand, circuit parameters which control only the rate of discharge through the valve, the forward and return strokes of the sawtooth waveform generated by the circuit can be varied independently.

In order that the invention may be clearly understood and readily carried into effect, the invention will be described with reference to the accompanying drawing, the single ligure in which illustrates one example of a sawtooth Waveform generator according to the present in- .ition.

Referring to the drawing, reference 1 denotes a capac- Jr across which, in operation of the circuit the desired ,awtooth Waveform is generated, the waveform being derived from output terminals 2 and 3. The circuit comprises three thermionic valves 4, and 6 which, though shown as triodes, may be pentodes in a practical circuit. The valve 4 functions alternately as the valve of a Miller integrator and as a discharge valve for the capacitor 1, whereas the valves 5 and 6 form part of the switch which conditions the valve 1 for its alternate functions. The valves 5 and 6 are coupled by resistors 7, 8, 9 and to form a conventional bistable D.C. coupled trigger circuit. The resistor 7 is connected to the terminal of a positive H.T. polarising supply. The anode of the valve 5 is connected to the anode of the valve 4 via resistor Patented July 28, 1959 11 and unilaterally conductive device 12, which may be a crystal diode. The capacitor 1 is connected between the anode and control electrode of the valve 4, and the control electrode is connected to the terminal of a negative H.T. supply by an integrating resistor 13, and to a source of negative bias voltage 14, of (say) -10 volts, by a diode valve 15, which may be a crystal diode. The cathode of the valve 4 is connected to the negative H.T. terminal by resistor 16, and to ground by unilaterally conductive device 17, which may be a crystal diode. The control electrode of the valve 5 is connected to the junction of resistors 18 and 19 which are connected from the anode of the valve 4 to the negative HIT. terminal. In practice, a cathode follower may be inserted to isolate the load of the resistors 18 and 19 from the capacitor 1. Other cathode followers may also be inserted in the circuits.

As shown, the circuit is free running and the two phases of each cycle, in which the forward and return strokes of a sawtooth Wave are respectively generated, are controlled by the two conditions of stability of the trigger circuit. In the rst phase, the valve 5 is cutolf, and its anode potential is maximally positive. The valve 6 is conducting, taking the common cathode potential of valves 5 and 6 to a higher potential than that at the control electrode of the valve 5. The capacitor 1 charges via resistor 11, diode 12 and resistor 13, the current in the resistor 11 exceeding that in the resistor 13 by the amount of current taken by the valve 4. The valve 4, the capacitor 1 and the associated components function as a self-regulating Miller circuit, the current in the valve 4 varying so as to tend to maintain the charging current for the capacitor 1 constant. During this phase, the potential at the control electrode of the valve 4 remains above the bias potential 14 so that the diode 15 is non-conducting. Moreover the current in the valve 4 is suicient to maintain the diode 17 conducting, maintaning a low cathode-to-ground impedance. The rise in potential at the output terminal 2, due to the charging of the capacitor 1, causes the potential at the control electrode of the valve 5 to rise runtil that valve conducts.

When the valve 5 conducts, its anode potential falls, and currents in the valve 6 and the diode 12 are cut-off. The potential at the control electrode of the valve 4 then falls until the diode 15 conducts. This fall in potential at the control electrode of 4 is accompanied by an equal change at 2, where a step is produced in the sawtooth waveform. Thereafter, the current is supplied to the resistor 13 only through the diode 15 and the control electrode of the valve 4 is maintained at a fixed potential. This potential brings about such a reduction in the cathode current of the valve 4, that the cathode potential falls and renders the diode 17 non-conducting. This establishes the conditions for the second phase of the cycle, during which the capacitor 1 discharges through the valve 4, to generate a return stroke, the discharge path comprising the elements 15, 1, 4 and 16. The cathode potential of the valve 4 is now self-regulating to set the control electrode-to-cathode potential to a value suitable for the current in the resistor 16. The valve 4 is thus in a constant current condition, by virtue of the cathode circuit feedback provided by the resistor 16, which is suitably dimensioned for this purpose. As the capacitor 1 discharges, the potential at the control electrode of the valve 5 falls until eventually the valve 6 conducts and initiates the first phase of the next cycle.

From the foregoing description, it would be understood that the anode of the valve 5, the negative H. T. terminal and the voltage lsource 14, constitute first, second and third points of reference potential for the valve I4. The

resistor 11, the diode valve 12, the capacitor 1 and the tween the diode 12 and the capacitor 1 on the one hand and between .the capacitor 1 and the resistor 13 on the other hand. The trigger circuit including the valves 5 and 6 constitutes a switch and in one condition of this switch the diode 12 is conducting and the diode l5 non-conducting `and in this condition, integration occurs in the capacitor 1 by Miller circuit action. In another condition of the switch the diode 12 is non-conducting and the diode 15 is conducting thereby tying the input electrode of .the valve 4 to the potential of the third reference point. In this condition of the switch the valve 4 functions as the conventional discharge valve for the capacitor 1, linearization of the discharge current being achieved by the resistor i6.

It will be appreciated that the constant current conditions during the two phases of a cycle can be varied`v independently of each other, by varying the values of the resistors 13 and 16, or by varying the potentials at the lower ends of these resistors. If the latter expedient is adapted, means are required for varying the potentials at the lower ends or resistors 13 and 16 independently of each other and of the negativeH. T. supply for other parts of the circuit, which should not be varied. rIherefore the slopes of .the forward and return strokes of the Even in the free running condition of the circuit synchronisation can be achieved by injecting triggering pulses at the cathodes of the valves 5 and 6.

The value given to the resistance 11 must be such as to limit the current fed to the anodeA of the valve 4 during the charging phase. This current must exceed that required to charge the capacitor 1 by an amount which can be regulated by the valve 4. Excessive current will overload the valve 4 giving excess charge to the capaci-tor 1. A deiiciency of current will'reduce the current in the valve 4 to a value below that required to keep the diode 17 conducting, and the control of the valve 4 will then ',CeaSe.

a switch having one condition in which said unilaterally conductive device is conducting and said capacitor is operative to provide negative feedback from said output electrode to said input electrode thereby to cause the Ageneration of a forward stroke `of a sawtooth waveform by integration in said capacitor, said switch having another condition in which said unilaterally conductive device is non-conducting and said capacitor is inoperative to provide negative feedback, thereby to cause the generation of a return stroke of said waveform by disgenerated sawtooth waveform can be varied independently. Moreover the duration of a return stroke canbe greater than that of a forward stroke. By varying the capacitance of thc capacitor 1, the slopes of both the forward and return strokes can be varied. ln the drawing the components 1, 13 and 16 are all shown as-variable indotted lines at 26 and 21. In this way the amplitudes j of the potential excursions elsewhere in the circuit are limited also and the amplitude of the generated sawtooth is made eiectively independent of stray circuit capacities.

In order to establish the rst waiting condition current is fed to the junction of resistors 8 and 9 to raise the mean con-trol electrode potential of .fthe valve 6. This holds the valve 5 non-conducting, even when charging of the capaci-tor l has ceased due to the anode potential of the valve 4 having reached its upper limit, set by one of the added diodes (the upper limit diode 20). suspends the cycle at the positive tip of lthe sawtooth. Triggering of the circuit to give one cycle of operation can then be effected by applying a negative potential excursion to 4the cathodes of the valves 5 and 6 sufficient to turn on the current in the valve 5.

The second waiting condition is established by bleeding current from the junction of the resistors 8 and 9 to lower the mean control electrode potential of the valve 6. This holds the valve 6 non-conducting even when the discharge of the capacitor l has ceased because the diode l2 is passing current from the anode of the valve 5, the potential at which is limited by the other -added diode (the lower lirnit diode 21). This suspends the cycle at the negative tip of the sawtooth. Triggering of the circuit to give one cycle of operation can then be eected by applying a positive potential excursion to the cathodes ofthe valves 5 and 6, sufficient to turn olf the current in thevalve S.

This

charge of the capacitor 4through said amplifier.

2. A sawtooth waveform generator comprising an ampliier having an input electrode and an output electrode, first, second and rthird points of reference potential, a path connected from said rst point lto said second point, said path including a rst conductive impedance, a unilaterally conductive device, a capacitor and a second conductive impedance arranged in that order, said output electrode being connected to said path between said unilaterally conductive device and said capacitor and said input electrode being connected to said path between said capacitor and said second conductive impedance, a second unilaterally conductive device connected from said third point n to said path between said capacitor and said second conductive impedance, and a switch for switching the potential at one of said points alternatively between a relatively high value and a lower value, said values being predetermined in relation to the potential at said other points to render the first and second unilaterally conductive devices conducting and non-conducting respectively in a rst condition ot said switch thereby to condition lthe capacitor for negative feedback from said output electrode to said input electrode and cause the generation of a forward r.stroke of la sawtooth waveform by integration in said capacitor, `and to render said rst and second unilaterally conductive devices non-conducting and conducting respectively in a second condition of said switch thereby to discontinue said negative feedback and `cause the generation ofthe return stroke of said ywaveform by discharge of said capacitor through said ampliiier. v

3. A generator according to claim 2, said switch comprising two valves each having an input electrode and -an `output electrode, and means cross-coupling the input and output electrodes to said valves, one end of said path being connected to the output electrode of one of said valves* wherebyv said output electrode constitutes one of said points of reference potential.

4. A generator according to claim 3 further comprising a coupling from the output electrode of said amplitier tofone valve of said switch, whereby transitions of the switch between its two conditions are responsive t0 the potential at the output electrode of said amplifier.

5. A generator according to claim 3, further comprisstroke of the sawtooth waveform, and means for applying triggering signals to induce transitions of said switch between its two conditions.

6. A generator according to claim 2, said amplifier having an electrode common to the input and output electrode circuits of said amplifier, a third conductive mpcdance connected from said common electrode to the second point of reference potential, a fourth point of reference potential and a third unilaterally conductive device connected from said common electrode to said fourth point, the potential at said fourth point being predetermined in relation to the potential at said other points to render the third unilaterally conductive device conducting in the rst condition of the switch to by-pass said third impedance, and to render the third unilaterally conductive device non-conducting in the second condition of the switch to cause said third conductive impedance to provide negative feedback for linearising the discharging current through said amplier.

7. A sawtooth waveform generator comprising an amplifier having an input electrode and an output electrode, a path including a capacitor connected from said output electrode to said input electrode, an integrating impedance for -applying potential to said input electrode, an output impedance for applying potential to said input electrode, and a switch having one condition in which said impedances are enabled to cause the generation of one stroke of a sawtooth waveform by integration in said capacitor,

and having another condition in which said impedances are disabled for integration and the capacitor is conditioned for discharge through said amplifier.

8. A sawtooth waveform generator comprising an ampliiier having an input electrode, an output electrode and an electrode common to the input and output circuits of said amplier, a path including a capacitor connected from said output electrode to said input electrode, an integrating impedance for applying potential to said input electrode, an output impedance for applying potential t0 said output electrode, and a feedback impedance for applying potential to said common electrode, a switch having one condition for shunting said feedback impedance and for enabling said integrating and output impedances to cause the generation of one stroke of a sawtooth waveform by integration in said capacitor, and having another condition for disabling said integrating and output impedances for integration and for conditioning the capacitor for discharge through said amplifier linearised vby said feedback impedance.

References Cited in the le of this patent Electronic Engineering, September 1948, The Miller Integrator, Briggs, pp. 279-284.

Article: A Rapid Flyback Miller Time Base Circuit, by Fraser et al., page 71 of Electronic Engineering, vol. 26, No. 312 for February 1954. 

